A striking characteristic of the human acute and chronic leukemias is the finding that they are the result of nonrandom, somatically acquired chromosomal translocations. The long-term goal of our research is to determine if there is a common mechanism involved in the generation of chromosomal translocations. As a biological model, we are using the RUNX1 gene, which encodes an important hematopoietic transcription factor and is the most frequent target of chromosomal translocations in acute myeloid leukemia (AMI). The RUNX1 gene is located on chromosome 21, and interestingly, for both the (8;21) and (16;21) AML-related translocations, all the genomic breakpoints are found in intron 5. Therefore, a key question is what are the factors that predispose the RUNX1 gene to be involved in chromosomal translocations? Our working hypothesis is that the chromatin structure present at the breakpoint regions is determinant for the t(8;21) translocation. Moreover, we hypothesize that the sub-nuclear localization of a gene may be determinant both of the gene susceptibility to dsDNA breaks and in the selection of the partner gene when a translocation is generated. A detailed comparison between the structural characteristics of chromatin and the nuclear positioning of the RUNX1, ETO and the recombinant AML/ETO locus will allow us to identify the presence of common features that may account for the increased accessibility observed at the breakpoint regions. The proposed experiments based on our preliminary data will combine the expertise of the collaborating laboratories to investigate the cis and trans acting factors that can be determinants of the RUNX1 locus recombination frequency. The focus of the parent grant is the interrelationship between chromatin organization, nuclear compartmentalization of Runx factors, and gene expression. In this FIRCA proposal, we extend the scope of these studies to address the relationship between Runx regulatory factors, nuclear localization, and susceptibility to chromosomal translocations. This research will be done primarily in Chile at the Universidad de Concepcion in collaboration with Soraya Gutierrez, as an extension of NIH grant 5P01 AR48818, projects.